This invention is related to mechanical pumps for moving or pumping metal such as aluminum or zinc in a bath of molten metal, and more particularly to such a pump in which a motor supported above the bath drives a vertical stainless steel shaft. The lower end of the shaft drives the impeller to create a stream of molten metal. A ceramic sleeve shields the stainless steel shaft to protect it from the corrosive effects of the heated molten metal, as well as forming a loose fit with the shaft to accommodate differences in the thermal expansion characteristics between the ceramic and the stainless steel.
Mechanical power driven pumps for moving metal in a bath of molten metal conventionally have a relatively short life because of the destructive effects of the molten metal on the pump components. If the pump shaft connecting the motor to an impeller is formed of any steel to provide sufficient torque to move the impeller in the molten metal, the steel has a short life because it is chemically attacked by the molten metal. If the steel shaft is shielded by a protective coating of a ceramic material, then the different thermal expansion characteristics of the steel and the ceramic causes the ceramic to shatter in a relatively short time.
A shaft made of graphite alone will burn at the metal surface. A shaft made of ceramic alone does not have sufficient tensile, torque or impact strength to overcome the stresses normally encountered when pumping molten metal.
A pump housing submerged in molten metal and made of graphite or ceramic material to withstand the heat, tends to rise in the metal bath because the ceramic has a lower density than the metal. In order to prevent the pump housing from rising in the metal, it is desirable to mount a series of vertical legs between the pump housing and an overhead supporting structure. In addition the legs (or posts as they are also called) should be strong enough to overcome the tensile stresses created during installation and subsequent removal of the pump in the molten metal bath. Such legs experience problems similar to that of an unshielded pumping shaft, that is, if they are made of an uncoated steel they have a short life because the steel is attacked by the molten metal. If they are made entirely of graphite, the legs will burn at the metal interface. If a leg is made entirely of a ceramic material having good heat resistant characteristics, it has insufficient tensile strength to ensure a long life.